In today's semiconductor technology there is an increasing demand for still smaller structures resulting in faster devices and allowing for higher packaging densities. For electronic and optoelectronic devices this has lead to the development of buried heterostructures in which the active layer, serving e.g. as a FET channel or as the light emitting layer in a diode laser, is embedded in a material providing for electrical and/or optical confinement.
Classically, such buried structures have been realized using crystal growth processes requiring two or more steps with intermediate lithography and etching. However, since the interfaces are, prior to the second step, exposed to air or etchants, causing oxidation and contamination, and because a substrate cleaning process is not possible without destroying the created pattern, defect generation at these interfaces is a problem since yield, reliability and controllablity are seriously effected.
Therefore, one-step growth of buried heterostructures on patterned substrates has attracted much attention. When using, e.g., Liquid Phase Epitaxy (LPE) or Metal Organic Vapour Phase Epitaxy (MOVPE) processes, due to different growth mechanisms certain planes (such as (111) and (011)) act as no or low-growth planes for specific compounds (e.g., GaInAs) whereas other compounds (e.g. InP) grow non-selectively on all planes. This allows the in-situ burying of, e.g., low-gap material (GaInAs) into wide-gap material (InP) without contamination of the heterointerfaces.
For certain device designs, selective lateral epitaxial growth of semiconductor material in a one-step growth process is desirable. For LPE, such processes have been described in a number of publications for which the following are representative:
"Growth Effects of InGaAs on InP Structured Substrates" by N. Chand et al. (Electr. Lett., Vol. 18, No. 14, July 1982, pp. 613-614);
"Single Mode InGaAs/InP Buried Waveguide Structures grown on Channelled (111)B InP Substrates" by T. M. Benson et al. (Electr. Lett., Vol. 1 8, No. 19, Sept. 1982, pp. 812-813);
"LPE Growth Effects of InP, InGaAs, and InGaAsP on Structured InP Substrates" by N. Chand et al. (J. Crystal Growth, 61, 1983, pp. 53-60);
"Novel High-Speed InGaAs/InP Lateral Phototransistor" by N. Chand et al. (Electr. Lett., Vol. 21, No. 7, March 1985, pp. 308-310).
These references describe selective lateral growth of InGaAs and InGaAsP on sidewalls of channels etched into low-growth (111)B-oriented InP substrates. The selective growth occurs on the rounded channel sidewalls of undefined orientation. The use of such methods is limited because of the inherent limitations for small, high-speed devices. The active layer, grown on a rounded sidewall, is geometrically undefined and is not suitable for high-performance sub-micron devices. In addition, the highly super-saturated LPE process does not allow the growth of well-defined sub-micron epitaxy layers. The undefined geometry of the layers also prevents use in devices requiring the growth of closely-spaced multiple active layers, e.g., for laser array structures.
More recently, weakly super-saturated growth processes such as MOVPE, VPE or CBE have been developed. They allow the selective, crystal orientation dependent deposition of very thin layers, down to the sub-nm range. The state of the art is represented by the following articles:
"Selective Epitaxial Growth of GaAs by Low-Pressure MOVPE" by K. Kaman et al. (J. Cryst. Growth, 73, 1985, pp. 73-76);
"A Study of the Orientation Dependence of Ga(Al)As Growth by MOVPE" by S. Hersee et al. (J. Cryst. Growth, 77, 1986, pp. 310-320);
"A Novel Technology for Formation of a Narrow Active Layer in Buried Heterostructure Lasers by Single-Step MOCVD" by A. Yoshikawa et al. (IEEE J. Quantum Electronics, 23, June 1987, pp. 725-729);
"A Novel MOVPE Technology for a Single Stage Growth of Buried Ridge Double Heterostructure Lasers and Waveguides" by M. Scott et al. (MOVPE Conference Japan, 3. Cryst. Growth, 93, 1988, p. 820);
"Buried GaInAs/InP Layers grown on non-planar Substrates by one-step low-pressure Metal Organic Vapor Phase Epitaxy" by Y. Galeuchet et al. (Appl. Phys. Lett., Vol. 53, No. 26, Dec. 1988, p. 2638), and by the
International Patent Application PCT-A-WO 87/00694 "Method for Producing a Heterostructure Device".
In these references growth processes for producing buried layer structures are described using selective growth techniques that allow one-step processes. In all tests made and for the applications proposed, the substrate is oriented such that the growth plane, normally (100)- oriented, is parallel to the initial unstructured horizontal substrate surface whereas the no-growth surfaces, mostly (111)-oriented planes revealed during a pre-epitaxial etching step, are inclined forming an angle with the horizontal substrate surface. In the resulting structures, the vertical dimension of the buried active layer is determined by an epitaxial growth step which can be accurately controlled. However, the process for determining the lateral, horizontal dimension either involves lithographic steps with their inherent limitations or requires complicated, low-yield processes virtually preventing the realization of well defined sub-micron buried structures in a production line. It has been found that these processes are, in fact, non-operational for very small dimensions (Article "Fabrication of Nanometer Width GaAs/AlGaAs Microelectronic Engineering 6 (1987), pp. 163-168). These processes are, furthermore, not suited for the fabrication of closely spaced multiple buried structures that are highly desired for a range of advanced high-performance devices.